SummaryAn NAC-type transcription factor gene AtNAC2 was identified from Arabidopsis thaliana when expression patterns of the genes from a microarray analysis were examined. The AtNAC2 expression was induced by salt stress and this induction was reduced in magnitude in the transgenic Arabidopsis plants overexpressing tobacco ethylene receptor gene NTHK1. AtNAC2 is localized in the nucleus and has transcriptional activation activity. It can form a homodimer in yeast. AtNAC2 was highly expressed in roots and flowers, but less expressed in other organs examined. In addition to the salt induction, the AtNAC2 can also be induced by abscisic acid (ABA), ACC and NAA. The salt induction was enhanced in the ethylene overproducer mutant eto1-1, but suppressed in the ethylene-insensitive mutants etr1-1 and ein2-1, and in the auxin-insensitive mutant tir1-1 when compared with that in wild-type plants. However, the salt induction of AtNAC2 was not significantly affected in the ABA-insensitive mutants abi2-1, abi3-1 and abi4-1. These results indicate that the salt response of AtNAC2 requires ethylene signaling and auxin signaling pathways but does not require ABI2, ABI3 and ABI4, intermediates of the ABA signaling pathway. Overexpression of AtNAC2 in transgenic Arabidopsis plants resulted in promotion of lateral root development. AtNAC2 also promoted or inhibited downstream gene expressions. These results indicate that AtNAC2 may be a transcription factor incorporating the environmental and endogenous stimuli into the process of plant lateral root development.

Ethylene signaling plays important roles in multiple aspects of plant growth and development. Its functions in abiotic stress responses remain largely unknown. Here, we report that alteration of ethylene signaling affected plant salt-stress responses. A type II ethylene receptor homolog gene NTHK1 (Nicotiana tabacum histidine kinase 1) from tobacco (N. tabacum) conferred salt sensitivity in NTHK1-transgenic Arabidopsis (Arabidopsis thaliana) plants as judged from the phenotypic change, the relative electrolyte leakage, and the relative root growth under salt stress. Ethylene precursor 1-aminocyclopropane-1-carboxylic acid suppressed the salt-sensitive phenotype. Analysis of Arabidopsis ethylene receptor gain-of-function mutants further suggests that receptor function may lead to salt-sensitive responses. Mutation of EIN2, a central component in ethylene signaling, also results in salt sensitivity, suggesting that EIN2-mediated signaling is beneficial for plant salt tolerance. Overexpression of the NTHK1 gene or the receptor gain-of-function activated expression of salt-responsive genes AtERF4 and Cor6.6. In addition, the transgene NTHK1 mRNA was accumulated under salt stress, suggesting a posttranscriptional regulatory mechanism. These findings imply that ethylene signaling may be required for plant salt tolerance.

The involvement of calcium and calcium-activated calmodulin (Ca 2ϩ -CaM) in heat shock (HS) signal transduction in wheat (Triticum aestivum) was investigated. Using Fluo-3/acetoxymethyl esters and laser scanning confocal microscopy, it was found that the increase of intracellular free calcium ion concentration started within 1 min after a 37°C HS. The levels of CaM mRNA and protein increased during HS at 37°C in the presence of Ca 2ϩ . The expression of hsp26 and hsp70 genes was up-regulated by the addition of CaCl 2 and down-regulated by the calcium ion chelator EGTA, the calcium ion channel blockers LaCl 3 and verapamil, or the CaM antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide and chlorpromazine. Treatment with Ca 2ϩ also increased, and with EGTA, verapamil, chlorpromazine, or trifluoperazine decreased, synthesis of HS proteins. The temporal expression of the CaM1-2 gene and the hsp26 and hsp70 genes demonstrated that up-regulation of the CaM1-2 gene occurred at 10 min after HS at 37°C, whereas that of hsp26 and hsp70 appeared at 20 min after HS. A 5-min HS induced expression of hsp26 after a period of recovery at 22°C after HS at 37°C. Taken together, these results indicate that Ca 2ϩ -CaM is directly involved in the HS signal transduction pathway. A working hypothesis about the relationship between upstream and downstream of HS signal transduction is presented.Organisms have developed a diverse array of mechanisms for adapting to environmental changes. One of the best characterized responses is the induction of heat shock proteins (HSPs). The heat shock (HS) response has been found in almost every organism studied to date. The HSPs are synthesized by cells in response to elevated temperature but are also induced by other environmental stresses (Noven et al., 1992; Kilstrup et al., 1997) and play an important role in the thermotolerance of plants (Queitsch et al., 2000; Burke, 2001). A connection between HS response and oxidative stress has been observed (Gong et al., 1997a;Lee et al., 2000;Larkindale and Knight, 2002; Panchuk et al., 2002). The HSPs are divided into several families based on their molecular mass, and most have molecular chaperones functions (for review, see Boston et al., 1996;Miernyk, 1999). Angiosperms synthesize more small HSPs (smHSPs) than other organisms. These smHSPs are likely critical for survival of heat stress and for specific developmental processes in plants (Waters et al., 1996).The changes in cytoplasmic calcium levels act as a ubiquitous signal in eukaryotic cells. HS induced a large increase in intracellular free calcium ion concentration ([Ca 2ϩ ] i ) in Chinese hamster (Cricetulus barabensis) HA-1 fibroblasts (Calderwood et al., 1988). In plants, Gong et al. (1998) observed that HS caused a transient increase in [Ca 2ϩ ] i . The change in [Ca 2ϩ ] i is also involved in regulating the binding activity of the HS transcription factor (HSF) to the HS element (Mosser et al., 1990), the synthesis of HSPs (Kiang et al., 1994;Kuznetsov et al., 1998), and...